1. Field of the Invention
The invention relates to a system for spacecraft stabilization and attitude control and, more particularly, to a flywheel based system for these purposes.
2. Description of the Related Art
The ability to stabilize a spacecraft in orbit and to reposition it as necessary is of great importance. Without this capability, most satellites would be all but dysfunctional. There have been a variety of systems developed to provide spacecraft attitude control, with the majority using some form of gyroscopic device. The initial approach was to use a series of stationary mounted gyroscopes each mounted to the spacecraft in such manner that the individual torques produced were orthogonal to one another. These gyroscopic devices, commonly known as reaction wheel assemblies (RWA) or momentum wheel assemblies (MWA), are commercially available from a variety of sources such as Honeywell, s Inc. Such assemblies are described in Honeywell's brochure entitled "Reaction Wheel and Momentum Weel Assemblies," April 1993. In operation, the stationary RWA/MWA units generally rotate at a near constant speed. A determination that the spacecraft is to be repositioned can be made internally by a spacecraft's on board computer system or by a ground based controller. Systems on board the spacecraft determine the direction and magnitude of movement as well as the amount of torque each of the units will have to generate to accomplish the repositioning. Torque is generated by the RWA/MWA units by either speeding up or slowing down a flywheel spinning within the unit, resulting in a change in momentum. This change in momentum generates the torque provided to the spacecraft, causing the spacecraft to move in the desired direction.
RWA/MWA units provide a reliable, cost effective way to generate spacecraft torques. However, they are only able to produce low levels of torque output, on the order of 1.6 Newton-meters (N-m) or less.
As a result of this shortcoming, systems have been developed to increase the torque output. One such system known as a momentum wheel platform (MWP), is described in U.S. Pat. No. 5,112,012 to Yuan et al. The MWP consists of an RWA/MWA unit mounted to a triangular shaped plate. Mounted to the corner of the plate are a series of jack screw legs which are controlled by independently operated stepper motors. The screws move up and down, causing the platform to tilt. The tilting of the platform, coupled with the torque generated by the RWA/MWA unit, results in an increased torque output. However, the jack screws can not move fast enough or far enough to produce the desired high torque levels for the time durations necessary in certain spacecraft designs.
To produce high levels of torque output, on the order of 305 N-m or more, for large, rapidly positioned space-craft, a system know as a control moment gyroscope (CMG) was developed. This type of system is commercially available from a variety of sources such as Honeywell, Inc., and described in Honeywell's brochure entitled "Control Moment Gyroscopes," April 1993. The control moment gyroscope consists of a spherical shell rotor spun at 5,000 to 6,000 rpm. The shell rotor is mounted within a single or multiaxis gimbal. Torque is generated by rotating the spinning shell rotor about one or more of the gimbal's axes. The system can produce high levels of torque output, and is capable of being rotated a full 360.degree.. However, they are quite large, approximately a meter in diameter, quite heavy, weighing 53 kilograms or more, and quite costly.
Due to its complexity, it is not as reliable as other torque producing systems and it has a high minimum weight which prevents it from being effectively scaled down.